A cementitious mixture that is comprised of class C fly ash and other industrial waste product such as either bottom ash or economizer ash. The mixture is preferably 15-35% weight class C fly ash based on the total weight of the mixture. The mixture excludes Portland cement, or other similar cement products. The dry mixture is placed in a desired location or form. Water is then added to the mixture when desired in order to harden the mixture in place. The cementitious mixture is particularly useful for securing fence posts, or the like, in the earth as the workability of the mixture is maintained by adding the water to the mixture only after the position has been properly plumbed and placed in the hole. The cementitious mixture thus provides an inexpensive alternative to standard blended cement products, and provides a convenient and effective way of securing fence posts to the earth.